Host-Pathogen-Biocontrol Agent Interaction as Affected by Sequential Application of Na2CO3 and CaCl2

Following the endured use of a restricted number of fungicides, the citrus industry is facing the outbreak of Penicillium digitatum (citrus green mold) and Penicillium italicum (citrus blue mold) resistant biotypes in the packing houses. Among the alternatives to synthetic postharvest fungicides, encouraging, but insufficient results have been reported for biocontrol agents. The efficacy was significantly improved when these agents were combined with GRAS compounds or physical means, but the mechanisms involved are still not clear. The understanding of the host-pathogen-biocontrol agent interactions as affected by the combined treatments is fundamental to further improve the treatment efficacy. With this in mind we studied the interaction among oranges (Citrus sinensis cv Washington navel) inoculated with P. digitatum and treated in sequence with a 3% solution of Na2CO3 (SC) and one containing 1% of CaCl2 with or without the antagonistic yeast (Pichia guiliermondii isolate 5A). Along with the direct effect of SC on P. digitatum pathogenesis, the induction of phytoalexins by SC and by the yeast played a key role in controlling pathogen growth. Also the application of CaCl2 following SC resulted very important indeed, the application of this salt improved significantly the growth rate of the antagonist and improved the mechanical barrier to pathogen penetration. The results will be discussed trying to shed light on the different interactions and make conclusions strengthened by some scanning electron microscopy observations.